go/src/cmd/compile/internal/ssa/copyelim.go

// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package ssa

// copyelim removes all uses of OpCopy values from f.
// A subsequent deadcode pass is needed to actually remove the copies.
func copyelim(f *Func) {
	// Modify all values so no arg (including args
	// of OpCopy) is a copy.
	for _, b := range f.Blocks {
		for _, v := range b.Values {
			copyelimValue(v)
		}
	}

	// Update block control values.
	for _, b := range f.Blocks {
		if v := b.Control; v != nil && v.Op == OpCopy {
			b.SetControl(v.Args[0])
		}
	}

	// Update named values.
	for _, name := range f.Names {
		values := f.NamedValues[name]
		for i, v := range values {
			if v.Op == OpCopy {
				values[i] = v.Args[0]
			}
		}
	}
}

// copySource returns the (non-copy) op which is the
// ultimate source of v.  v must be a copy op.
func copySource(v *Value) *Value {
	w := v.Args[0]

	// This loop is just:
	// for w.Op == OpCopy {
	//     w = w.Args[0]
	// }
	// but we take some extra care to make sure we
	// don't get stuck in an infinite loop.
	// Infinite copy loops may happen in unreachable code.
	// (TODO: or can they? Needs a test.)
	slow := w
	var advance bool
	for w.Op == OpCopy {
		w = w.Args[0]
		if w == slow {
			w.reset(OpUnknown)
			break
		}
		if advance {
			slow = slow.Args[0]
		}
		advance = !advance
	}

	// The answer is w.  Update all the copies we saw
	// to point directly to w.  Doing this update makes
	// sure that we don't end up doing O(n^2) work
	// for a chain of n copies.
	for v != w {
		x := v.Args[0]
		v.SetArg(0, w)
		v = x
	}
	return w
}

// copyelimValue ensures that no args of v are copies.
func copyelimValue(v *Value) {
	for i, a := range v.Args {
		if a.Op == OpCopy {
			v.SetArg(i, copySource(a))
		}
	}
}